Car has new battery (less than a month old), has 12.2 V at the posts, can unlock/lock via the fob (and can open trunk), inside lights turn on, get in push the start button - dash comes alive, push on the brake and the light on the start button turns green, hit the start button again - EVERYTHING does dead except for the tach. Tach goes nuts and there is a noise from the engine when the tach goes haywire. The key light on the dash has a ! on it. Changed batteries with our CX5 fob (again - ONLY changed the fob battery) and it did not help.

My only assumption at this time is that the fob is broken... Anything else? Same basic action if I use the end of the fob to hit the start button. It is weird as I would expect the light on the start button to only turn green if the fob was recognized, which would seem to sat the fob is ok.

I don't believe with this year and model Mazda has the key access in the steering column to use the key.

 

Your battery voltage min should be no less then rested 12.50 volts. Just because the battery is new is only based on when your purchased it but not the actual age of the battery. I always suggest having the battery load tested. Especially with your voltage.
Next to check the battery terminals connection and especially at the ground point on the frame/chassis.
If the dash light is acting as you described it suggests more to be a connectivity issue which when you do the first part I suggested then if that did not cure the no start we can move to the next suggestion.

 

This might help... video.

Terminals are good and tight and not corroded. I have a noco and once that is good and charged then I will try and jump. Car is driven mostly on longer drives - 20-30 miles to work for my son - each way - so even if not fully charged from the store - the alternator should be charging it to full capacity in a couple days max.

 

Remeasured the voltage at the posts - 12.54 and 12.6 the 2 times. I probably just remembered it incorrectly before. Removed the bolt for the ground wire at the driver side wheel housing and cleaned it up and the end of the cable - really clean to begin with any how, but did not have any effect.

 

Ok well will say for now that the battery is ok and the connections are good??

So the next step is to check your immobilizer. Let's assume it is good battery. You need to use you spare key fob that is not damaged. If you don't have one, then this is likely beyond a DIY at this point to either find or to correct the no start.
You can review by reading up on a Mazda authored service or owner's manual publication of the immobilizer.

 

Unfortunately, we don't have another key - car was bought used and with only 1 fob. We have a local dealer so I can either get a new fob from them or have them diagnose.

Thanks for the suggestions - appreciate it.

 

You may check with your local Mobil locksmith services.

mobile locksmith southwest Ohio - Search

 

Ok - was going to have it towed to the dealer as they were pretty sure it was not a fob issue - and I would have needed to get 2 to program. So - went out to prepare the car for towing and noted - I could not get the gear selector into neutral (automatic unfortunately). Popped the cap of the gear selector lock and got it into neutral, and on a whim tried to start it - and started right up.

Really odd - not sure what to make of it but they are going to take a look at it and see if there are contacts that are dirty or switch having issues.

 

If it started you first order should be an additional key ASAP before you have another no start issues that possibly was caused by your damaged one?

 

I will look into getting a second one - the mobile key service and the deal are about the same in price. I think I will do some searching to see if a dealer parts dept does what a BMW dealer/part dept does and offer a fob for my 2001 BMW for about half the price as new.

They replaced the brake pedal switch - really cheap and hopefully - should fix the issue. If it happens again will have to remember trying the gear lever and putting the car into neutral.